You can try to clean the project if it happens (Ctrl + P) and then run again.
Can you post the full error message from the logs?
Ctrl+P didn't solve the problem. Here is the full error message.
Program started.
Error occurred on line: 5461
java.lang.IllegalArgumentException: Can not set static anywheresoftware.b4j.objects.MenuItemWrapper$ConcreteMenuItemWrapper field b4j.example.main._mnulogin to anywheresoftware.b4j.objects.MenuItemWrapper
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
at sun.reflect.UnsafeStaticObjectFieldAccessorImpl.set(UnsafeStaticObjectFieldAccessorImpl.java:79)
at java.lang.reflect.Field.set(Field.java:764)
at anywheresoftware.b4a.shell.Shell.setStateBeforeUserSub(Shell.java:379)
at anywheresoftware.b4a.shell.Shell.raiseEventImpl(Shell.java:287)
at anywheresoftware.b4a.shell.Shell.raiseEvent(Shell.java:158)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at anywheresoftware.b4a.BA.raiseEvent2(BA.java:93)
at anywheresoftware.b4a.ShellBA.raiseEvent2(ShellBA.java:90)
at anywheresoftware.b4a.BA.raiseEvent(BA.java:84)
at b4j.example.main.start(main.java:36)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$159(LauncherImpl.java:863)
at com.sun.javafx.application.LauncherImpl$$Lambda$53/483920725.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$172(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl$$Lambda$45/1915503092.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$170(PlatformImpl.java:295)
at com.sun.javafx.application.PlatformImpl$$Lambda$48/599658954.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$171(PlatformImpl.java:294)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/1567581361.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$145(WinApplication.java:101)
at com.sun.glass.ui.win.WinApplication$$Lambda$36/1645995473.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)